In the oil and gas industry, geophysical survey techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon or other mineral deposits. Generally, a seismic energy source, or “seismic source,” generates a seismic signal that propagates into the earth and is partially reflected and refracted by subsurface seismic interfaces between underground formations having different acoustic impedances. The reflections are recorded by seismic detectors, or “receivers,” located at or near the surface of the earth, coupled in the ground by any method such as cementing or placement in bentonite, in a body of water, or at known depths in boreholes, and the resulting seismic data can be processed to yield information relating to the location and physical properties of the subsurface formations. Seismic data acquisition and processing generates a profile, or image, of the geophysical structure under the earth's surface. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of them.
Various sources of seismic energy have been used to impart the seismic waves into the earth. Such sources have included two general types: 1) impulsive energy sources and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. In the second type of geophysical prospecting, a vibrator is used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. Except where expressly stated herein, “source” is intended to encompass any seismic source implementation, both impulse and vibratory, including any dry land or marine implementations thereof.
The seismic signal is emitted in the form of a wave that is reflected and refracted off interfaces between geological layers. The reflected and refracted waves are received by an array of geophones, or receivers, located at the earth's surface, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by means of recording equipment. The receivers typically receive data during the source's energy emission and during a subsequent “listening” interval. The recording equipment records the time at which each reflected and refracted wave is received. The seismic travel time from source to receiver, along with the velocity of the source wave, can be used to reconstruct the path of the waves to create an image of the subsurface. A large amount of data may be recorded by the recording equipment and the recorded signals may be subjected to signal processing before the data is ready for interpretation. The recorded seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations. That information is then used to generate an image of the subsurface.
Oil and gas reservoirs may be continuously surveyed to provide real-time monitoring of the reservoir. A continuous seismic monitoring system may consist of an array of receivers located near the reservoir and one or more sources. The sources continuously operate to emit a seismic signal. The receivers receive the reflected and refracted signal, which is recorded by recording equipment and then transmitted to a data processing location for storage and processing.
The data acquired by the recording equipment may be transmitted in several ways. In some systems, data is transmitted to the data processing location using a physical link, such as a buried cable. However, digging trenches and laying cable may be expensive, may interfere with surface or subsurface improvements such as roads, pipes, and other cabling, and may raise environmental concerns. In some systems, data is transmitted to the data processing location using a wireless link. However, the volume of data acquired by the recording equipment may be large compared to the transmission rates for the wireless link, which is often much lower than that for a physical link. In addition, the recording equipment may be battery-powered, which limits the amount of time the equipment can spend transmitting per day. Thus, it would be useful to provide methods and systems to reduce the amount of data sent to the data processing location over a wireless link.